Winding core for heat shrinkable film material

ABSTRACT

A generally cylindrical core for winding a web of sheetlike material has portions with a lesser diameter than other portions such that otherwise baggy areas of the web overlie the lesser diameter area of the core. The core particularly solves the problem of elongated edges on a wound film of heat shrinkable thermoplastic material caused by differential shrinkage about a core.

BACKGROUND OF THE INVENTION

This invention relates generally to the art of winding a web ofsheetlike material and more particularly to a new winding core for usein handling such material.

In the art of producing thermoplastic film, many complex and involvedmeasures have been taken in order to wind such film about a core toproduce a roll with a substantially uniform profile. One of thesetechniques is disclosed in U.S. Pat. No. 2,844,846. The techniquedisclosed therein comprises extrusion of the thermoplatic film into abubble prior to passing the tubular extrudate through nip rolls. Theextrusion die is rotated relative to the nip rolls such that anyimperfection which might be created from the die configuration will beevenly distributed about a roll resulting from the film. The evendistribution of slight imperfections is an extremely important aspect ofwinding thermoplastic film. When such imperfections are allowed toconcentrate and pile upon one another, the magnitude of a slightimperfection becomes greatly exaggerated and magnified after severalthousand windings. By evenly distributing such imperfectionstransversely across a roll such magnification is avoided. Anothertechnique for producing a roll of thermoplastic film material having auniformly distributed imperfections is disclosed in U.S. Pat. No.3,161,942. The process disclosed therein comprises rotation of thewinding roll rather than rotation of the extrusion die.

Another technique for improving the profile of the rolled sheet of filmis disclosed in U.S. Pat. No. 3,052,917. The process disclosed thereincomprises pulling the edge portion of film from the die at a more rapidrate than the central portion. The net effect of this process is thepulling of wrinkles and distortions from the film prior to rolling.

While all of these techniques confront the problems associated with theplacement and winding of a sheet of thermoplastic film about a core ormandrel, certain phenomina occur once a heat shrinkable film is woundabout a core. Heretofore the prior art has not confronted the problemsassociated with film distortions which occur after the winding of thefilm onto a core for storage in handling.

SUMMARY OF THE INVENTION

It is thus an object of this invention to provide a core for winding aweb of sheetlike material into a roll which minimizes distortions in thewound roll.

It is a further object of this invention to provide a core for windingthermoplastic film material which minimizes distortions which occurafter winding.

It is a further object of this invention to provide a core for winding aweb of heat shrinkable thermoplastic film material.

It is a still further object of this invention to provide a process forwinding a web of thermoplastic film material which minimizes distortionswhich occur after the film has been wound on a core.

It is yet a further and more particular object of this invention toprovide a process for winding a web of heat shrinkable film materialwhich minimizes film distortion caused by film shrinkage on the woundroll.

These as well as other objects are accomplished by winding a web ofmaterial about a core which has a lesser diameter in areas where the webotherwise tends to bag or sag when unrolled.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is an isometric drawing of winding core in accordance with thisinvention.

FIG. 2 is an isometric drawing of a core wound with heat shrinkable filmin accordance with this invention.

DETAILED DESCRIPTION

According to this invention, the problems associated with web shrinkageand/or distortion during storage on winding cores or mandrels has beengreatly minimized. In accordance with this invention it has been foundthat heat shrinkable web material will distort on a winding core duringstorage to produce areas which are baggy and generally non-uniform inlength. Such non-uniformity greatly hampers subsequent efforts toutilize the film with high speed production equipment such as thatutilized in the form, fill and seal processes.

In accordance with this invention it has been found that suchnon-uniformity can be overcome by utilizing a winding core which has alesser outer diameter beneath the wound web in areas where it is desiredfor the resulting roll to have a lesser longitudinal expanse afterstorage. It has surprisingly been found that baggy or sagging areas areeliminated if the film is stored on rolls which have a reduced outerdiameter in areas where such sagginess tends to occur. In the preferredform of this invention a core having terminal areas which have a lesserouter diameter than the central portion is utilized. Such a winding coreis generally shown in FIG. 1 of the drawings.

This invention in actuality has a dual aspect. It is directed broadly tothe winding of a web of sheetlike material. In one aspect the inventionis directed to solving problems of distortion created by the shrinkageof a shrinkable film during storage. In another aspect it is directed towinding a roll which has distortions prior to winding which tend tocreat objectionable areas of bagginess during the unwinding thereof. Thefollowing description is given with emphasis to the first but it isunderstood that the inventive concept is equally applicable to thesecond aspect.

As used within this disclosure the term heat shrinkable film is utilizedto mean thermoplastic film which has been formed by an extrusionprocess. All film is heat shrinkable to some extent. The term "heatshrinkable" in its more common usage is utilized to refer to film whichhas been oriented by techiques well known to the art. Thermoplasticfilm, however, exhibits a certain amount of heat shrinkability evenwithout purposeful orientation. The problem which this inventionconfronts and solves are more readily apparent on oriented film. Theinvention, however, is applicable to heat shrinkable film which is notoriented. This invention is thus applicable to the more common filmssuch as polymers of the following monomers: the mono-olefins andconjugated di-olefins, e.g., ethylene, propylene, butene-1, isobutene,1,3-butadiene, isoprene and other aliphatic mono and di-olefins; thehalogen substituted olefins, e.g., vinyl chloride, vinylidene chloride,;the monovinyl aromatic compounds, e.g., styrene, alpha methylstyrene,chlorostyrene, and other aromatic olefins; and other ethylenicallyunsaturated monomers such as acrylonitrile, acrylamide and the like.

By this invention it has been found that the prior art practice ofwinding heat shrinkable film into a generally cylindrical core with auniform outer diameter produces a resulting roll of film which is not assuitable for storage for significant periods of time as is altogetherdesirable. It has been found that such prior art rolls permit film toshrink transversely during storage such that the edge areas thicken,particularly at the outer convolutions of the roll, to force the edgeportion of the roll to a larger diameter than prior to such shrinkage.The edge portions take a set at this larger diameter. Other areas of thefilm do not shrink because of the general immobility thereof caused bythe mutual trapping of the individual convolutions. The net effect ofthis transverse shrinkage on the outer convolutions of the roll is thatthe edge portion of the outer convolutions thicken due to the transverseshrinkage, thus forcing the outer edge portions of the roll to a largerdiameter and thus increasing the longitudinal expanse of the edgeportion. The edges which are unwound from each convolution are thuslonger than the central portion of the film of that convolution.Bagginess or sagging of the edges makes the film generally difficult tohandle and particularly difficult for feeding to high speed packagingequipment. The problem is even further complicated by disproportionatebagginess in the two edge portions. This disproportionate bagginessresults in edge portions which have length differentials.

According to this invention the problems associated with baggy orsagging film portions is solved by winding the thermoplastic film abouta core which has a reduced outer diameter in an area where bagginesstends to exist. The bagginess of the edge portions of heat shrinkablethermoplastic film is thus eliminated by winding the film about a coresuch as that shown in FIG. 1 of the drawing. In such an instance theedge portions of the film are wound about the terminal portions 1 and 3of the core 5. The resulting roll of material appears in FIG. 2 of thedrawings. As is apparent therein, the edge portions of the film, 7 and9, lie on a smaller circumference than the central portion 11 of theroll. The shrinkage of the film during storage thus results in edgeportions which are generally shorter than the central portion of thefilm. While this does result in some non-uniformity of longitudinalexpanse, the length differential does not create a problem duringprocessing by high speed equipment since the edge portions are theportions fed to such equipment. By pulling on the shortest part of thefilm there is no problem associated with bagginess. It has surprisinglyand additionally been found that the tendency to have differentiallengths in the two edge portions is minimized by utilizing the windingcore in accordance with this invention.

Another advantageous feature of the core and film wound thereon by thetechnique of this invention is that any longitudinal variations ordistortions may be compensated for by stretching only the edge portionof the film since this is the shortest longitudinal part of the film. Inthe prior art longitudinal distortions could be corrected only bystretching the entire width of a sheet of film with a consequential lossof uniform characteristics. This beneficial result of the subjectinvention is totally surprising and unexpected.

While this invention is described with particular reference to solvingthe problem of edge bagginess produced during storage of heat shrinkablefilm, it is readily apparent that the technique employed by thisinvention may be utilized to solve the problem of any distortion whichmight be found undesirable during storage of a heat shrinkable plasticfilm material. In some instances it thus may be found desirable toeliminate bagginess in a central portion of a thermoplastic sheet offilm material. In such an instance the outer diameter of the core isdecreased in the central portion of the core. The decrease in diameterin the appropriate area is preferably of sufficient magnitude to matchthe thickening of that portion when a core of constant diameter isutilized. Thus if bagginess occurs from a roll having an outer radiusdifferential of 0.1 inch, then the problem is solved in accordance withthe preferred embodiment of this invention by utilizing a core with anouter radius in that area which is 0.1 inch less than the remainingradius. In general any variations in thickness are within the range of0.1 to 10 percent of the diameter of the resulting roll.

By a similar analysis the edge portions of the roll illustrated in FIG.1 are of sufficient size to encompass the area of the edge portion whichwould develop sags or bagginess during storage. It has generally beenfound that from about 1 to 20 percent of the film width should overliean area of reduced diameter at each edge. About 5 percent of the film,width at each edge has been found to be an optimum for mostapplications. The amount of reduced diameter is best determined, asdescribed above, by matching the decrease in diameter to the increasewhich would otherwise occur on a roll of constant diameter. While agradual lessening in outer diameter is preferred, a core having adiscontinuous abrupt change of outer diameter may also be utilized.

Having fully disclosed the invention, the following example is given asa further illustration and aid to the understanding thereof.

EXAMPLE

A film of oriented polypropylene 22.5 inches wide was wound on thecenter of a 3 inch diameter core which was 23.3 inches long to afinished diameter of 91/2 inches (8,700 feet of film). The centralportion of the film was 18.5 inches wide lying on a diameter which was0.06 inches greater than the diameter of the terminal portions. The rollwas aged for 7 days at 115° to 120° F. This caused the outer convolutionto shrink 3/8 of an inch transversely. Upon unwinding no edge bag wasobservable. Normally edge bag would have developed to a degree whichwould have made the roll non-functional. A similar roll of orientedpolypropylene wound on a conventional core would have a shrinkage ofabout 3/4 of an inch after such storage. The extent to which thetransverse shrinkage was reduced was unexpectedly surprising.

While this invention has been described with particular reference to theuse of a core for solving problems associated with the shrinkage of heatshrinkable film during storage, it is readily apparent that theinvention is applicable to solving problems associated with distortionsin webs of sheetlike material which are present prior to winding on aroll. Thus a web which is distorted to have thicker areas along atransverse section thereof may be wound to a uniform outer diameter bywinding the thicker area over a core having a reduced diameter in thearea of the thickened portion. The invention is also applicable tosolving problems associated with materials which tend to shrink on aroll from phenomena other than heat shrinkage. Textile materials such aswool and other natural or artificial fibers which tend to shrink withtime may be wound on rolls which have sections of reduced diameter inaccordance with the teachings of this invention. In any instance theamount of diameter reduction on a particular roll is preferably matchedby the increase in diameter which would occur had a conventional corehaving a uniform outer diameter been used.

It is thus seen that a novel process for winding and storing webs ofsheetlike material is provided by the process of this invention. A novelcore for winding heat shrinkable material is also provided within thescope of this invention. Many other advantages and uses thereof will beapparent to those skilled in the art from a reading of the abovedescription. Such other uses, however, are embodied within the scope ofthis invention as is measured by the appended claims.

What is claimed is:
 1. A roll of heat shrinkable thermoplastic film,comprising:a generally cylindrical core having a central portion andterminal end portions, said end portions having a gradually lesser outerdiameter than said central portion of said core; and convolutions ofsaid film about said core, each of said convolutions comprising edgeareas which would otherwise tend to shrink in an axial direction andthus elongate circumferentially after winding and during storage, saidareas which tend to elongate overlying and contacting said terminal endportions of less outer diameter after storage.
 2. The roll of heatshrinkable film according to claim 1 wherein said heat shrinkable filmis a polymer of a mono-olefin or a conjugated di-olefin.
 3. The roll ofheat shrinkable film according to claim 2 wherein said heat shrinkablefilm is a polymer or copolymer of a monomer selected from the groupconsisting of ethylene, propylene, butene, isobutene, butadiene, andisoprene.
 4. The roll of heat shrinkable film according to claim 1wherein said heat shrinkable film is a polymer of a halogen substitutedolefin.
 5. The roll of heat shrinkable film according to claim 4 whereinsaid halogen substituted olefin is selected from the group consisting ofvinyl chloride and vinylidene chloride.
 6. The roll of heat shrinkablefilm according to claim 1 wherein said heat shrinkable film is comprisedof an aromatic olefin.
 7. The roll of heat shrinkable film according toclaim 6 wherein said heat shrinkable film is a polymer or copolymer of amonomer selected from the group of styrene, alpha methyl styrene, andchlorostyrene.
 8. The roll of heat shrinkable film according to claim 1wherein said film is a polymer of ethylene.
 9. The roll of heatshrinkable film according to claim 1 wherein said shrinkable film is apolymer of propylene.
 10. The roll of heat shrinkable film according toclaim 1 wherein said shrinkable film is a copolymer of propylene andethylene.